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Our of car has sat unused for long enough the battery was totally dead. We jumped it the other day and drove around for 30min or so, but a few days later it is too flat to start again.

From this level, how long do I need the car running to charge it to near full so I can be confident it will start?

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Depending on how long the battery has sat around, it may not be able to take a charge any longer due to sulfation of the lead plates within the cells. You need to take the battery out of the vehicle, put it on a charger at a slow rate (2A or less) and let it charge overnight (or longer). When through, take it to a battery shop which can test it for you. They can show you what the useful life is on the battery. The battery has a voltage to maintain (lead-acid batteries in cars are usually 13.1-13.5vdc when fully charged at rest) as well as a capacity (or storage of electrical power). A battery can charge up to the 13vdc, but not be able to maintain that voltage for very long when being used.

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